In section 2.2 I reviewed the definition
of cD galaxies as supergiant
elliptical-like galaxies with very extensive outer halos. The classic
and most extreme examples are brightest galaxies in rich clusters.
They are clearly distinguishable from ordinary ellipticals both because
they are so much brighter
(Oemler 1976;
Dressler 1978,
1979;
Thuan and Romanishin 1981)
and because their halos are enormous compared to halos of ellipticals (e.g.,
Morgan and Lesh 1965;
Oemler 1973).
However, the
transition to ordinary ellipticals is smooth. Therefore it is not
immediately clear how we should classify "cD galaxies in poor clusters"
(Morgan, Kayser and
White 1975;
Albert, White and
Morgan 1977),
the brightest galaxies NGC 4874, 4889 and 4839 in the Coma cluster, or even
M87. What are the quantitative properties of cD
galaxies, and to what
extent do they imply physical differences from ordinary ellipticals?

The central parts of cD galaxies are superficially similar to very
bright ellipticals. Their profiles satisfy r1/4 laws
(Thuan and Romanishin
1981)
or Hubble laws
(Oemler 1973,
1976).
Photometric
parameters of cD galaxies in rich clusters are also normal: they fall on
the extrapolation toward higher luminosity of the
Be - logre relation
for ordinary ellipticals
(Fig. 13). The same
conclusion has been reached by
Hoessel (1980,
see Fig. 17) from his photometry of 108
first-ranked cluster ellipticals, many of which are cDs.
Thuan and Romanishin
(1981,
Fig. 10) find that cD galaxies in poor clusters also
fall on the brightward extrapolation of the Be -
logre relation. In
fact, their Hubble-law parts have the same luminosities as corresponding
parts of cDs in rich clusters. The "reduced magnitudes"
Mred = - 2.5 log(I0a2) + constant for nine cDs in rich clusters
(Oemler 1976)
have an average value of <Mred> = -20.58
(dispersion = 0.40). Nine cDs in poor clusters have
<Mred> = -20.69 (dispersion = 0.56).
Thus the main bodies of cD galaxies in rich and poor clusters are
photometrically indistinguishable. Both are indistinguishable from
ordinary ellipticals except for the fact that on average they are more
luminous. cD Galaxies extend the magnitude range of the
Be - log re
relation for ellipticals by about 2 mag to
-19 MB -25
(H0 = 50 km s-1 Mpc-1).

One very important observation distinguishes first-ranked cluster
ellipticals, including cDs, from ordinary ellipticals. Close doubles
and multiple nuclei are surprisingly common considering their short
lifetimes before they merge into one object
(Matthews, Morgan and
Schmidt 1964;
Morgan and Lesh 1965;
Jenner 1974).
The best-known
example is NGC 6166, which has four nuclei very close to each
other in projection deep inside the light distribution
(Minkowski 1961).
Another impressive object which cannot long survive unchanged is
V Zw 311, which has at least nine components
imbedded in a common halo
(Zwicky 1971,
p. 102;
Gunn 1977b).
Hoessel (1980)
has shown that ~ 28% of first-ranked cluster galaxies have multiple
nuclei. These
observations constitute the best evidence that galaxy mergers play an
important role in the formation of cD and other first-ranked galaxies
(e.g.,
Ostriker and Tremaine
1975;
see Tremaine 1981
and White 1982 for
reviews). If this is so, then the similarity of cD and elliptical
galaxies implies either that ordinary ellipticals also form through
mergers, or that merger remnants are photometrically indistinguishable
from other ellipticals.